2025 Theses Doctoral

Wildfires, Aridity, and the Pacific: Drivers of enhanced wildfire activity across the western United States since the 1980s

Juang, Caroline

Wildfire in the western United States (US) is a natural disturbance that has become a destructive force impacting human life, human infrastructure, and ecosystems. Since 1984, the western US has experienced a ~250-300% increase in annual area burned and >1000% in forest area burned. A significant portion of the trends are driven by warming and drying from a combination of anthropogenic climate change and internal climate variability. A century of fire suppression has resulted in abundant fuel to burn, and the current amount of forest area burned is well below pre-industrial amounts. If climate is the main driver of burned area in the western US, how will climate variability and change continue to influence burned area across the western US? This is a guiding question for this Ph.D. dissertation and the field of study.

The goal of this Ph.D. is to expand our scientific knowledge of the teleconnected and physical drivers of burned area, from anomalous sea-surface temperatures (SSTs) to the flame front, utilizing the most up-to-date observations of climate, burned area, and SSTs; statistical modeling; and global atmosphere model outputs. We contribute to this field by answering three questions: (1) What drives annual forest area burned to respond exponentially to aridification? (2) How did the observed La Niña-like strengthening of the east-west tropical Pacific sea-surface temperature (SST) gradient contribute to western US wildfire extent in 1984–2022? (3) How have SSTs, internal atmospheric variability, and anthropogenic warming influenced western US burned area?

In Chapter 1, we find that annual area burned will continue to increase exponentially with linear increases in atmospheric warming and drying, due to the ability of individual fires to spread exponentially at their fireline when fuel is available. Annual burned area trends are dominated by the largest fires that have the greatest potential to grow exponentially in response to increased aridity—the warmth and dryness of the atmosphere.

In Chapter 2, we use this relationship in statistical models to predict annual are burned and link the role of observed Pacific SST patterns contributed by the Pacific Decadal Oscillation (PDO). We find that the observed tropical Pacific west-east sea-surface temperature (SST) gradient had a small but important role in enhancing the positive burned area trend since 1984, and was responsible for 20% of the 1984–2022 increase in wildfire extent.

In Chapter 3, we further explore the effect of SSTs on western US wildfire by forcing our burned area model with climate simulated by an atmospheric model that was forced with observed SSTs. Consistent with the results of Chapter 2, we find evidence that SST variability played a minor role in dictating western US burned area, explaining 18% of the variance, but more work is needed to disentangle the effects of SSTs from the effects of anthropogenic climate change. However, the simulations also suggest that the combination of observed SSTs and internal atmospheric variability had the potential over the past four decades to promote occurrences of annual burned areas that far exceed that of 2020, the current record holder.

Throughout this work, we demonstrate the importance of the relationships among SSTs, western US hydroclimate, and historical and future area burned. Our findings should motivate continued work to reduce uncertainty regarding how past and future anthropogenic forcing will affect tropical Pacific ocean-atmosphere coupling and associated climate teleconnections globally. Our findings should also promote continued preparedness for the likelihood of rapid increases in western US forest-fire activity that outpace current expectations and investment in remote sensing and in situ technologies that track wildfire activity in the western US.